If I understand correctly, however, the term is slightly misleading as it implies that the children's actual age is taken into account. In reality the ARE are the same for all children in the same school year, no matter if they're e.g. 6+10 months at the end of the school year, or 7 + 9 months.In fact they are not 'age' related at all, but rather 'school year' related expectations.

Typically ARE is for the academic year as brilliotic explained however you may find that they do take the child's age into consideration, especially if they are working above or below the ARE. We certainly do in Early Years as it makes a huge difference if you have a child born in Sept compared to a child born in August (obviously we hope and support every child to reach the ARE but it's another factor to consider when looking at progress).